This project has two potentially related goals. The first goal is to identify the cellular and molecular interactions that stimulate immature thymocytes to develop. The second goal is to investigate how these interactions may be involved in the development of malignant thymic tumors. These studies will provide insight into the development of the immune system and into how this process becomes disordered in malignancy. The experimental design will develop a unique in vitro model to define what cell interactions and signals stimulate immature tumors to proliferate and differentiate. These studies will then be extended to analyze the growth requirements of tumors in vivo. Other experiments will extend these studies to analyze the development of normal thymocytes.